The process of the present invention is applicable to any cellulosic fibrous material but is described with particular reference to the bleaching of wood pulp, preferably wood pulp produced by the Kraft process, i.e., wood pulp produced by digestion of wood chips in the pulping liquor containing sodium hydroxide and sodium sulfide as the active pulping chemicals. Following the wood digestion process, pulp is separated from the spent pulping liquor. The spent pulping liquor is then recovered and regenerated for recycling.
The Kraft process wood pulp is then bleached and purified in a bleach plant operation. In the bleach, plant, pulp is first subjected to oxidizing chemicals such as chlorine and/or chlorine dioxide and followed by extraction with a suitable source of alkali such as sodium hydroxide. Depending on the desired pulp brightness, one to three additional bleaching stages are employed with typically one being an alkali extraction stage and one or two stages using oxidizing chemicals. Following each bleaching stage, spent bleaching chemicals are removed from the pulp by washing with a suitable source of water; either fresh water or previously used water from pulp washing or a combination of the two. Current state of the art requires that all wash water from the bleach plant along with spent bleaching chemicals be discharged to the sewer as effluents rather than being processed in the pulping liquor regeneration operation noted above. Various concerns have prevented the recovery of these bleach plant effluents in the pulping liquor regeneration operation. One concern is the possible build-up of chlorides causing corrosion and operational problems. Another concern is the lack of adequate systems for removing non-process metals, such as calcium, magnesium and manganese, which enter the bleach plant and are typically removed with wash water and spent bleach chemicals.
Further, the use of large volumes of water for pulp washing in the bleach plant has also prevented recovery of bleach plant effluents due to the resulting high evaporator load. The environmental impact of these practices has been widely noted, but despite these serious concerns, the ability to overcome the problems associated with recovery as described above has not been developed.
In the 1970's, the closed mill concept for recovering bleach plant effluents developed by Rapson, U.S. Pat. No., 3,698,995, and later improved by Reeve et al. in U.S. Pat. No. 4,039,372 was tested at Great Lakes Papers in Thunder Bay, Ontario. The mill began operation in 1977 but was abandoned in 1985. The mill attempted to reduce the discharge of the bleach plant effluent by recycling the filtrates to the pulping liquor regeneration process. The bleaching process employed was (D/C)EDED. The process included a salt crystallization system as a means of purging sodium chloride. The chloride levels introduced into the pulping liquor regeneration process were within the range of 90-100 lbs/adt pulp. With the crystallization process, the concentration in white liquor was equal to or in excess of 100 grams per liter before equilibrium was reached, i.e., chloride entering is equal to the amount of chloride removed.
This process was unsuccessful in the recovery of the bleach plant filtrates. The reasons for this were related to continued concerns of corrosion, pulp quality, inadequate removal of non-process metals and high evaporation requirements.
After the experience at Thunder Bay, Reeve noted the following features as needing to be developed for the success of future effluent-free mills: low chloride concentrations in the recovery process; a chloride removal system employing minimal evaporation; low bleach filtrate flow to minimize evaporation requirements, a bleaching sequence that minimizes bleach chemical consumption due to recycle of extracted organic components; and adequate systems to remove minor wood components such as potassium, calcium and pitch.
Substituting chlorine dioxide for chlorine and incorporating oxygen delignification in the bleach sequence reduces the chloride content of the bleach effluent when compared to sequences without oxygen and chlorine dioxide. Use of the ODED sequence, referenced in U.S. Pat. No. 4,039,372, for recovering bleach plant filtrates would reduce the sodium chloride input to the recovery process by 73% over that experienced at Thunder Bay during closed mill operation. While use of the ODED bleach sequence would represent a significant reduction in chloride content of the bleach filtrate streams compared to Thunder Bay's closed mill experience, the resulting chloride content of black liquor feeding the recovery boiler would cause operating problems. The chloride content of black liquor feeding the recovery boiler is a function not only of the quantity of chloride introduced to the recovery process but also the process used to removed chlorides. A significant fraction of the chloride feeding the recovery boiler is the result of chloride accumulation in the ash that is volatilized in the boiler, collected and returned to the boiler feed. Potassium, which also affects recovery boiler operation, accumulates in the boiler ash in a similar manner. At Thunder Bay, sodium chloride was removed from the recovery process by evaporation and crystallization of sodium chloride from white liquor. This chloride removal process not only represents a significant increase in the evaporation load of a mill, it also does not prevent the large cyclic flow of sodium chloride and potassium compounds contained in the recovery boiler ash which represents a significant fraction of the chloride and potassium load to the recovery boiler.
U.S. Pat. No. 4,039,372 by Reeve et al. shows that 1895 gallons per minute of bleach filtrate is recovered from a 500 T/D bleach plant and sent to the pulp mill/recovery operation. All liquor recovered from the bleach plant must be evaporated. The flow of 1895 gallons per minute represents more than twice the normal volumetric flow needed for brown stock washing. According to U.S. Pat. No. 4,039,372, a portion of the 1895 gallons per minute is used for diluting white liquor that is concentrated during the sodium chloride removal operation. The net result is a nearly doubling of the evaporation load of the mill. The most significant factor contributing to the large filtrate flow from the bleach plant in U.S. Pat. No. 4,039,372 is the use of fresh water to wash pulp at two locations in the bleach plant.
Various metals such as calcium, magnesium, manganese and potassium enter the pulp mill with the wood supply. These metals if not adequately purged from the pulping and bleaching operations can cause operating problems. In current operations these metals are released from the pulp in the first acidic stage of the bleach plant due to the low pH (2-3) of operation and are purged to the sewer along with filtrate from this same first stage. Pulp is thoroughly washed as it leaves the first acidic stage of bleaching to prevent any entrained liquor containing solubilized metals from being carried into later stages of bleaching. If not adequately removed in the first acidic stage of bleaching, manganese and iron can affect bleaching in the later stages by limiting brightness development and increasing chemical consumption.
In U.S. Pat. No. 4,039,372, Reeve et al. included a fresh water wash on the first acidic bleaching stage washer which prevents possible carryover of metals to the later stages of bleaching. A second fresh water wash volume was used after the final stage of bleaching. In this manner, two wash volumes of fresh water must be evaporated as compared to evaporation of only one wash volume if bleach plant filtrates are not recovered.
Counter-current pulp washing with bleach filtrates can result in the accumulation of calcium and magnesium in the bleach plant/brown stock washing systems due to adsorption of metals on pulp at high pH and re-dissolving at low pH. This accumulation of calcium and magnesium can result in the deposition of inorganic and organic matter on pulp and equipment which can increase bleach chemical consumption and require down time for equipment cleaning. Reeve et al. described the problems of scaling and lignin deposition if calcium is not removed when practicing the art of recovering bleach filtrates. To avoid this problem Reeve et al. used a portion of the filtrate from the first acidic stage of bleaching in the causticizing plant thereby purging from the bleach plant some of the dissolved metals. This method of purging metals is limited by the volume of filtrate that can be used in causticizing. According to Reeve et al., approximately 325 gallons of the 1288 gallons of first acidic stage filtrate is purged to the causticizing operation or about 25 percent. The remaining 75 percent of the filtrate from the first acidic bleaching stage is used to wash pulp on the decker preceding the bleach plant allowing the possibility for metals to accumulate. Following the experience at Thunder Bay which incorporated this method of purging metals, Reeve noted that additional development would be needed to adequately purge metals for the closed mill concept to be feasible.
When recovering bleach plant filtrates, it is not possible to remove all dissolved organic matter entrained in the pulp from counter-current displacement washing with bleach filtrates, before pulp enters the bleach plant. This is particularly true if fresh water usage is to be limited to keep evaporation requirements to a minimum. Reeve published laboratory results documenting the increased bleach chemical consumption that results as the quantity of dissolved organic matter is increased in the first stage of bleaching of a D-CE sequence at 70% chlorine dioxide substitution. This study showed that bleach chemical consumption increased as dissolved organic matter, removed from pulp in the D-C stage, was re-introduced into the D-C stage, simulating the recovery of bleach plant filtrates. The same conclusions were reached when dissolved organic matter, removed from pulp during the extraction stage, was added to the D-C stage. Bleach chemical consumption increased more for extraction stage organic matter compared to D-C stage organic matter. Competition exists for bleaching chemicals in the first acidic stage of bleaching between dissolved organic matter and lignin within the pulp fibers. As the quantity of organic matter added increases, increasing amounts of bleach chemicals in the first stage are consumed non-productively by the dissolved organic matter. This results in less delignification of the pulp as measured by the pulp kappa number. Following the experience at Thunder Bay, Reeve noted the need for developing a bleaching sequence that minimized bleach chemical consumption due to dissolved organic matter carried into the bleach plant as a result of counter-current displacement pulp washing.
Recently published investigations on recovering chlorine-based bleach plant effluent have concluded that recovery of these effluents along with the pulping liquors in the conventional recovery process is not technically feasible. Others have published investigations on separate recovery operations for pulping liquors and bleach plant effluent.
No process has since been developed that comes close to a substantial recovery of bleach plant filtrates. In fact, based on a recent publication (Paper ja Puu--Paper and Timber; 5/89), several Pulp and Paper Research Institutes in Scandinavia concluded that closure of the bleach plant was not likely to be available technology for at least another decade and would be dependent upon significant developments in both pulp bleaching and in chemical recovery.
It would therefore represent a notable advance in the state of the art if a new process for recovering chlorine-based bleach plant effluents in a conventional pulping liquor recovery operation could be developed which provides very low chloride concentrations in the recovery process; little or no significant impact on evaporation requirements; more complete non-process metal removal; and negligible impact on bleach chemical consumption due to recovery of bleach filtrate.